At present, ring-type current transformers (“CTs”) are the most prevalent technology for measuring phase currents in three-phase electric power transmission and distribution lines. Conventional CTs are often placed in physical contact with the monitored power line conductor, which results in excessive heating of the CT and the power line. Excessive heating can adversely impact electronics in the CT and limit the current carrying capacity of the power line. Conventional CTs are also limited to electric current measurement, resulting in the need for separate voltage sensors when both current and voltage measurement are desired. Conventional CTs also require separate radios to transmit the current measurements to controllers or remote transmission units (RTUs). These radios are typically powered by batteries or separate low voltage wiring. Batteries require periodic maintenance, while low voltage wiring requires a nearby transformer, which increases the cost and maintenance requirements.
In addition to current measurement, power line voltage measurement is also useful for “smart grid” electric power system monitoring, protection and control. Traditional voltage measurement devices, while accurate, are expensive and physically large, which limits the number of electric power circuit locations where they can be cost effectively be deployed. High voltage sensors are presently available, but the conventional voltage sensing techniques are usually sensitive to electromagnetic interference errors (also referred to as “cross talk” or “contamination”) caused by other high voltage power lines and devices in close proximity. In addition, many conventional voltage sensors are non-directional, which renders them particularly susceptible to electromagnetic contamination. Electromagnetic contamination also impairs the measurement of the voltage phase angle, which is critical for VAR measurement, capacitor switching for voltage support, and sophisticated direction-to-fault and distance-to-fault sectionalizing techniques.
A high voltage sensor in the form of a circuit board carrying a foil patch sensor has been used to obtain advantages over physically large capacitor and transformer-based systems. The small size and low cost of foil patch sensors allows for information gathering at a larger number of measurement points, which provides more accurate and robust determination of power outages and poor power quality conditions. However, conventional capacitive foil patch sensors are typically non-directional, which makes them susceptible to significant cross talk from nearby high voltage power lines and devices. While a line mounted patch sensor with a ground plane positioned adjacent to the power line experiences relatively good coupling to the power line, nearby power lines and other high voltage devices, such as disconnect switches and equipment bushings, can still cause significant cross talk.
Conductive shielding and mathematical filtering are other techniques that have been used to avoid or compensate for electromagnetic cross talk. However, conductive shielding can be physically challenging to design and mathematical filtering is complicated by variations in capacitance caused by changing environmental factors. Voltage sensors are generally physically mounted to a support made from a dielectric medium, such as epoxy, Teflon or porcelain. While the nominal capacitances of these materials are generally well known, the actual capacitance in the field can vary significantly with changes in environmental temperature, moisture and surface contamination. For example, the sensor capacitance can change with changes in physical dimensions resulting from thermal expansion. Power line conductors experience physical sag, which can be significantly impacted by the ambient temperature. In addition, the intrinsic dielectric constant of some materials can change with temperature. While the majority of these effects can be compensated for by measuring the temperature of the local mechanical support and using a calibration table to account for these changes in capacitance, this increases the cost and complexity of the measurement system.
Moreover, temperature compensation may not be sufficient in some cases because sensor performance can also vary significantly due to the surface condition of the physical sensor support. Any type of electrically conductive surface contamination can drain the electric charge from the sensor and impact the phase angle response characteristic of the sensor. These parasitic effects are caused by a resistive layer, such as water, ice, oxidation or grime, on the surface of the sensor support or housing. Resistive surface contamination can significantly change the measured phase angle with respect to the voltage field and reduce the signal magnitude available to measure. Moisture or high humidity in the ambient air can exasperate these sensor impacts. While these contamination effects do not significantly influence the high voltage field itself, they do affect the signal measurement in both magnitude and phase angle.
Solar storms can cause geomagnetic disturbances that produce direct current (“DC”) currents in electric power transmission systems. These DC currents can saturate grounded transformer windings, which can overheat the transformers and cause voltage instability problems. This leads to power outages and component failure. Conventional CTs are unable to measure the DC component of power line currents, which prevents electric utility system operators from taking appropriate actions. Moreover, conventional protection relays are unable to react to DC current because they are only configured to respond to very high AC fault currents.
High resolution analog-to-digital converters are now available to measure DC currents in the presence of high power alternating current (“AC”) currents. Other techniques can be used to improve the accuracy DC current measurement. For example, the skin effect of DC versus AC current can be used to separate the AC current signal from the DC current signal. In addition, a 50/60 Hz notch filter with DC gain can be used to increase the accuracy of the DC measurement and flatten the frequency response of the conductor impedance. Another technique uses an AC-coupled signal to remove the DC component and then subtract the AC-only signal from the original. Averaging across the 50/60 Hz power cycle window can also be used to reject the fundamental AC frequency and leave the DC component.
However, these approaches are generally expensive, require complicated signal processing, and prone to cross talk errors from stray voltage sources. As a result, there is a persistent need for improved current and voltage sensors for high voltage power lines. There is a particular need for high voltage electric power line monitors capable of measuring DC currents, AC currents and voltages with onboard communication features suitable for placement in many circuit locations in smart-grid applications.